This invention relates in general to sintered ceramic bodies. In one respect, this invention is directed to polycrystalline sintered bodies based on silicon carbide which exhibit high fracture toughness and relatively lower brittleness and good chipping resistance. In a further aspect, the invention is directed to a process for the preparation of sintered bodies and the use of sintering additives to promote the formation of a transient liquid phase during the densification of silicon carbide.
Structural ceramic materials which retain their strengths to temperatures on the order of 1400.degree. C. to 1500.degree. C. are desirable for their application in high temperature environments including, for example, those encountered in automotive applications such as gas turbines, diesel superchargers, stirling engines and the like. Currently silicon carbide and silicon nitride are the leading contending materials for use in gas turbine engines. "SiAlON", an acronym derived from the chemical symbols of the constituents silicon, aluminum, oxygen and nitrogen, is a group of materials which are somewhat related to silicon nitride and which generally exhibit higher toughness, but lower strengths than silicon nitride, and higher strengths, but lower oxidation and creep resistance than silicon carbide.
Silicon nitride (Si.sub.3 N.sub.4) is typically densified, aided by a liquid glassy phase, at temperatures ranging from 1500.degree. C. to 1850.degree. C. for times which can be as short as 30 minutes. The presence of a liquid phase is critical to the process since it allows alpha phase silicon nitride to be converted into beta phase silicon nitride. The initially present alpha phase silicon nitride transforms into beta phase silicon nitride. This process produces an acicular microstructure (as distinguished from an equiaxed microstructure) which can provide good fracture toughness. Typical sintering aids used to densify Si.sub.3 N.sub.4 include MgO, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, ZrO.sub.2, CeO.sub.2, and CaO. These oxides and others react with silica present on the surface of the silicon nitride to form a glassy amorphous (noncrystalline) phase at temperatures below 1850.degree. C. This glassy phase tends to inhibit creep resistance at elevated temperatures of 800.degree. C. and above. Decomposition of Si.sub.3 N.sub.4 begins as low as 1000.degree. C. and becomes progressively greater as temperature is increased, becoming quite excessive at temperatures greater than 1700.degree. C. Although Si.sub.3 N.sub.4 materials can have a greater strength and toughness than conventional SiC and therefore can be more resistant to catastrophic failure, SiC has higher hardness and is therefore preferred in wear applications. Also, SiC has a higher resistance to creep which is beneficial in heat engine applications. Creep is that property of any material wherein deformation occurs at elevated temperatures either with an applied load or otherwise.
The chemical and physical properties of silicon carbide make it an excellent material for high temperature structural applications, such as gas turbine engine components. These desirable properties include excellent oxidation resistance and corrosion resistance, relatively high thermal conductivity compared to other ceramics, relatively low thermal expansion coefficient compared to metals, relatively high resistance to thermal shock and relatively high strength at elevated temperatures. For example, SiC is stronger than the nickel superalloys at temperatures above 1000.degree. C. and has better creep and oxidation resistance, as well as being potentially less expensive. Another advantage is that the theoretical density of silicon carbide, being 3.21 g/cm.sup.3, is less than half that of the superalloys. On the other hand, other characteristics of known bodies of essentially pure sintered silicon carbide, particularly those produced by known pressureless sintering processes, are considered undesirable, including inability to be electrical discharge machined (EDM) at an acceptable rate due to generally poor electrical conductivity (high electrical resistivity), high sensitivity of the microstructure to variations in sintering conditions, grain growth upon extended or repeated exposure to relatively high temperatures above about 1900.degree. C., and low (relative to SiAlON or silicon nitride) fracture toughness.
The sintering of silicon carbide, in the absence of applied mechanical pressure ("pressureless sintering"), has been accomplished using various sintering aids. Those sintering aids include carbon (C), boron (B) and compounds thereof, e.g., boron carbide (B.sub.4 C), and aluminum (Al), and compounds thereof, e.g., alumina (Al.sub.2 O.sub.3), individually and/or in combination. Such sintering aids have been used to obtain essentially nearly single crystalline phase silicon carbide with relatively high densities, for example, 97% of theoretical density or greater. "Sinter-active SiC powders", those having high specific surface areas of about 1 M.sup.2 /g to 100 m.sup.2 /g, and an average nominal diameter of about one micron or less, with some powders being less than 0.5 microns in average nominal diameter, are considered necessary to achieve relatively high densities, for example, as is described in U.S. Pat. No. 4,312,954.
The sintering of conventional silicon carbide (using boron and carbon as sintering additives) typically takes place at temperatures of at least 1900.degree. C. but below 2500.degree. C., and typically in a range of about 2100.degree. C. to 2250.degree. C., and is accomplished principally by solid state diffusion without the occurrence of a liquid phase; this is known as solid state sintering. Pressureless sintering allows for the economically viable commercial fabrication of complex shapes, and so it is desirable for silicon carbide particles to be densified without applied mechanical pressure, as well as at reduced temperatures, due to the fact that high temperatures are known to promote grain growth and a consequent deterioration in physical properties.
A drawback of conventional silicon carbide ceramic material is its brittleness, which, for example, causes the edges of a sintered body to be easily chipped in handling, resulting in poor production yields. Another drawback is sensitivity to the introduction of internal flaws during the production process, causing low strength values, rendering the resultant material undependable in respect to the application of localized high stresses during use. The known sintered silicon carbides, including the conventional materials, do not exhibit high enough toughness to overcome the loss of strength occasioned by such flaws, even when such flaws are relatively minor.
Silicon carbide densification by the pressureless sintering method, with sintering additives, has been the subject of various patents including the SiC--B--C system (U.S. Pat. Nos. 4,179,299; 4,004,934; 4,526,734; 4,692,418; 4,124,667; and 3,649,342) and the SiC--Al--C system, (U.S. Pat. No. 4,692,418 to Boecker, et al, 1979 and U.S. Pat. No. 4,230,497 to Schwetz, et al).
The SiC--B--C system provides different properties depending upon the amount of sintering additives and the particular production process used. These properties allow various applications of silicon carbide from armor tiles to electrical heating elements. However, the extremely high hardness and the transgranular fracture mode, inherent in such conventional silicon carbide, produce relatively low fracture toughness. The typical fracture toughness data for SiC--B--C system is between 4 and 5 MPam.sup.1/2 measured by the single edge notched beam (SENB) test method as is well known to those with skill in the art.
The SiC--Al--C combination is another well-studied system which also offers high density, good high temperature strengths and thermal stability. This material also exhibits a transgranular fracture mode at room temperature and intergranular fracture mode at elevated temperature. Oxidation resistance is reported to be better than that of the SiC--B--C system. Fracture toughness for this system is typically within the range of about 4 and 6 MPam.sup.1/2.
Silicon carbide has been sintered to high density, using rare earth oxides as additives, usually resulting in high strength. Omori, et al. U.S. Pat. Nos. 4,502,983, 4,564,490, and 4,569,921 disclose the use of rare earth oxides to promote solid state diffusion sintering. These inventions require the use of SiC of submicron size and typically result in surfaces having higher concentrations of rare earth oxides. High bend strength, between 59.6 ksi and 127.7 ksi are reported, however, it is not known whether or not these strengths are based on a three-point test method (which has been generally used in Japan and which generally gives values which are 30% to 50% higher than a four-point test) or based on the four-point bend test method which is more universally accepted and considered more representative and more reflective of accurate true bend strength. The Saito U.S. Pat. No. 4,681,861 teaches the use of Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --Cr.sub.2 O.sub.3 as sintering additives and reports very dense silicon carbide bodies. A bend strength of 42.6-66.7 ksi (by the four-point bend test method, but using 4 mm.times.3 mm.times.35 mm test pieces in accord with Japanese standard JIS R-1601) and indentation fracture toughnesses of 5-6 MPam.sup.1/2 were also reported, and the product had good corrosion resistance at high temperatures in contact with molten steel. Virkar, et al. U.S. patent application Ser. No. 778,251, which is disclosed in U.S. Pat. No. 4,829,027 to Cutler et al., discloses a method for densifying mixtures containing Si--C--Al--O--N into a solid state diffusion sintered body, as taught by U.S. Pat. No. 4,141,740, using a liquid phase provided by the carbothermal reduction of alumina (Al.sub.2 O.sub.3) to produce "SiCAlON". Such a technique is not believed to result in ceramic material exhibiting high fracture toughness.
Japanese published patent application ("Kokai") to Nagano, Application No. 51384/59 filed Mar. 16, 1984 and Publication No. 195057/60 published Oct. 3, 1985 discloses a system wherein Al.sub.2 O.sub.3, CeO.sub.2 and SiO.sub.2, in separate and distinct powder forms are mixed with SiC powder, formed with a temporary binder and sintered within a temperature range of 1800.degree. C. to 1950.degree. C. This prior art application teaches firstly that it is absolutely necessary to add SiO.sub.2 as a separate and distinct powder, secondly, that the sintered body will decompose at sintering temperatures above 1950.degree. C. causing the loss of shape of that body and a substantial reduction in bend strength and, thirdly, that the mechanism is entirely liquid phase. This prior art application, insofar as the sintering additives are concerned, also teaches that AlN can be substituted for Al.sub.2 O.sub.3 and that Y.sub.2 O.sub.3 can be substituted for CeO.sub.2.